To obtain a large-scale image, a projection display has been used which converts light beams from light sources into an optical image by a small light valve in accordance with a video signal, and projects the optical image under magnification on a screen (Japanese Patent Application Laid-Open No. 2006-235642; hereinafter referred to as JP2006-235642). JP2006-235642 discloses the construction of an illumination optical system provided with an optical integrator made up of two lens array plates when applied to a liquid crystal projector which is a type of projection display. JP2006-235642 also discloses an optical system which utilizes two light sources and combines light beams from the two light sources using prism array plates.
In the above-described projection display, a light beam (circular beam) from each light source is finely split into stripes by a prism array plate, and the respective split sections of the light beam are alternately arrayed in one direction, so that light beams from the respective light sources are combined. Accordingly, the composite light beam has an almost elliptical shape with its width in one direction twice that of the almost circular original beam. This requires of the prism array plate to have its longer side twice the length of its shorter side. When a positive lens is disposed behind the prism array plate, this optical lens also needs to have a diameter almost twice that required in the case of a single light source, generally arising the need to construct a large-scale optical system, which results in cost disadvantage.
A light valve such as a liquid crystal panel and a DMD (digital micromirror device) currently being used widely is defined to have a rectangular aspect ratio of 4:3, 5:4, 16:9 or the like depending on the application of a projection display. As the composite light beam produced by combining light beams from two light sources has an aspect ratio closer to such aspect ratio, the efficiency in light utilization increases. However, the projection display disclosed in JP2006-235642 and a projection display in which two light sources are disposed in parallel for connecting light beams from the two light sources (Japanese Patent Application Laid-Open No. 2005-346093; hereinafter referred to as JP2005-346093) both have an aspect ratio of about 2:1, which disadvantageously degrades the efficiency in light utilization.
Unlike the projection displays disclosed in JP2006-235642 and JP2005-346093 each processing parallel light beams from two light sources as they are, there is another type of projection display which combines light beams from two light sources upon correcting the angle of optical axes of the light beams from the two light sources using an optical element (deflection member), (Japanese Patent Application Laid-Open No. 2006-308778). In this projection display, the light beams from the two light sources are combined side by side, causing the composite light beam to have an aspect ratio of 2:1 similarly to the aforementioned JP2006-235642 and JP2005-346093, which disadvantageously degrades the efficiency in light utilization. In addition, since the light beams from the two light sources are combined upon being concentrated on a point (focal point), an optical element (deflection member) provided in that focal point needs to be made of a high heat-resistant material, which is also disadvantageous in terms of reliability and costs.